IABP balloon catheters are often used in medical treatment of heart diseases. In the treatment, a balloon catheter is inserted into the artery near the heart f the patient, and the balloon is inflated and deflated in synchronization with the heart beat of the patient in order to assist or activate the heart function. Japanese Patent Application Laid-open No. 60-106464 (hereinafter "JP '464") discloses a medical-purpose driving apparatus for inflating and deflating such balloons.
The driving apparatus disclosed in JP '464 has a primary tube line and a secondary tube line, which are isolated from each other by a pressure-transfer isolator (simply called an isolator, or generally named a volume limiting device (VLD)). A change of pressure occurring in the primary tube line is transferred to the secondary tube line via the isolator without allowing gas flow between the secondary tube line and the primary tube line, and the resultant pressure change in the secondary tube line causes the balloon to inflate and deflate. The reason why the primary and secondary tube lines are separated from each other is that different kinds of fluid are used in these two lines, namely, one has a driving gas for actually driving the balloon, the other has a source gas from which a positive pressure and a negative pressure are generated. This is required to improve the inflation/deflation response of the balloon and to keep the secondary tube line sealed in order to prevent gas leakage except the leakage due to diffusion. This arrangement is capable of generating a pressure at a low cost, while reducing consumption of an expensive fluid used in the secondary tube line. The pressure-transfer isolator is located between the pressure source and the balloon in order to prevent excessive gas from flowing into the balloon when inflating and deflating the balloon.
In this IABP balloon catheter, helium gas, which has a small mass and a high response ability, is preferably used as the fluid to fill the secondary tube line. In this case, the helium gas functions as a driving shuttle gas. However, because of its small molecular weight, helium gas diffuses through the tube wall or the balloon film even if there are no pin holes in the secondary tube line. If helium gas is enclosed in the sealed secondary tube line, and if the balloon is continuously driven for 20 minutes to 30 minutes, the gas pressure inside this tube line decreases by several millimeters (mm) of mercury (Hg).
For this reason, the helium gas must be appropriately supplied to the secondary tube line during the use of the balloon catheter. If additional helium gas is not supplied, the amount of helium gas inside the secondary tube line gradually decreases and, finally, the balloon cannot be sufficiently inflated, which means that the balloon cannot aid the heart function of the patient any longer. Some gas supply systems are known, which monitor the interior pressure of the secondary tube line using a pressure sensor, and which supply gas so that the detected pressure does not drop below a predetermined value. In these systems, the solenoid valve is opened certain times in a short amount of time to supply helium gas from the high-pressure gas cylinder via the helium gas tank in which the pressure is adjusted to a secure level.
However, if the helium gas is additionally supplied without limitation, and if a pin hole is formed in the balloon by accident, then a great amount of gas may flow into the blood vessel of the patient, which may cause gas obturation and may be fatal to the patient. Accordingly, in the conventional driving apparatuses, the pressure of the helium gas is kept relatively low using a mechanical regulator, and the gas is supplied to the secondary tube line several times separately. Also, a mechanism for monitoring the total number of opening/closing actions of the solenoid valves in the series of gas supply operations and the time interval between gas supply operations is provided for the purpose of regulating the gas supply.
However, in the conventional apparatuses, the amount of helium gas supplied per unit time cannot be accurately known even if the number of opening/closing actions of the solenoid valve is monitored. It is impossible for the conventional apparatuses to keep the same interior pressure of the helium gas tank before and after the switching of the solenoid valve. In addition, the switching time of the solenoid valve itself varies, and the balloon pressure is affected by the patient's blood pressure. For these reasons, the amount of helium gas supplied per opening of the valve or the number of opening/closing actions of the valve is not stable. Thus, it is difficult to accurately know the total amount of supplied helium gas based on the number of opening/closing actions of the solenoid valve. If a strict restriction is imposed on the number of switching actions of the valve and the time interval between gas supply operations, during which the interior pressure of the secondary tube line gradually decreases, then an improper alarm may be given due to the influence of fluctuation in the opening/closing time of the solenoid valve. On the other hand, if the monitoring restriction is loosened, it becomes difficult to distinguish abnormal gas leakage due to, for example, a pin hole formed in the balloon catheter or the secondary tube line, from the natural gas leakage (or diffusion) during the normal operation.
Even if the relationship between the number of switching actions of the solenoid valve versus time is placed on a chart, this chart does not help the operator (or the observer) distinguish abnormal gas leakage from natural gas leakage because the number of switching actions does not indicate the accurate amount of helium gas additionally supplied.
By the way, several types of balloon catheters for IABP having different volumes are generally prepared so as to meet with the patients' physiques. In the conventional apparatuses, when the balloon catheter connected to the secondary tube line is replaced with another balloon catheter having a different volume, the pressure-transfer isolator is also changed according to the size (volume) of the new balloon catheter. Some pressure-transfer isolators have a mechanism for adjusting the stroke of the diaphragm, which serves as the pressure transfer element, according to the size of the balloon catheter. In these isolators, the stroke of the diaphragm is varied as the balloon catheter is changed without replacing the isolator itself, so that the appropriate amount of driving gas is supplied to drive the new balloon catheter. Unfortunately, this type of driving apparatus is likely to become large because of its mechanical structure and, in addition, the operator must set the stroke of the diaphragm to an appropriate value each time the balloon catheter and the pressure-transfer isolator are changed. If a motor is used as the driving mechanism in such a pressure-transfer isolator, the response speed of the stroke is not fast enough. For these reasons, the pressure-transfer isolator itself is generally changed in the conventional driving apparatuses, rather than adjusting the stroke of the diaphragm.
If a pressure-transfer isolator, designed for a small volume balloon, is used to drive a large volume of balloon catheter, it cannot sufficiently aid the heart function of the patient. Conversely, if a pressure-transfer isolator, designed for a large volume balloon is used to drive a small volume of balloon catheter, excessive pressure is applied to the balloon, and danger of gas leakage increases.
In order to replace the pressure-transfer isolator without causing gas leakage, appropriate sealing structures must be provided to both the pressure-transfer isolator and the receiving part of the housing of the apparatus. These sealing structures must be tightened appropriately. This requires a high rigidity in the pressure-transfer isolator and the receiving part of the housing. Consequently, both the weight and the cost, of the entire apparatus, increase. Although a variable-stroke pressure-transfer isolator can keep its chamber airtight using fixed connecting means (e.g., screws or adhesive), it has drawbacks as explained above.
In general, rubber diaphragms used in pressure-transfer isolators are fatigued through use, and they must be replaced when the cumulative driving numbers approach the marginal value. However, since the pressure-transfer isolator is often changed according to the volume of the balloon catheter to be used, it is difficult to keep a record of the cumulative driving number of each diaphragm and to accurately specify the time for replacement of the diaphragm. The diaphragm of a pressure-transfer isolator is generally changed earlier in order to ensure safe operation, which further increases both the cost and the labor.
Japanese Patent Application Laid-open No. 5-10952 (hereinafter "JP '952") discloses a technique for driving a medical appliance, which does not require the pressure-transfer isolator to be replaced or adjusted even if a different volume of balloon catheter is used. In JP '952, the interior pressure of the secondary tube line (including tubes and hoses) on the balloon catheter side is monitored and, simultaneously, the balloon pressure is also detected at a timing of switching a driving state of balloon to a deflated state from a inflated state. A gas is supplied into the balloon side tube line so that the pressure (i.e., the plateau pressure) in the inflation state is kept constant. In this method, the interior pressure is automatically adjusted whenever a different volume of balloon catheter is used and thus, manual adjustment by the operator is not required.
However, this driving apparatus does not take a slight change of the balloon volume into account, which slight change in balloon volume may be caused by, for example, the fatigue of the balloon, inappropriate pressure application, bend of the blood vessel of the patient, or unexpected accidents (such as, the balloon getting stuck in swelling inside the blood vessel). Accordingly, the driving apparatus keeps on supplying helium gas to the tube even if the balloon volume changes because one of the situations explained above, occur. In addition to this, the life of the distorted balloon becomes shorter, which is undesirable.
In the apparatus of JP '952, the balloon is degassed when the blood pressure of the patient increases as the patient recovers, and when the blood pressure of the patient reaches a prescribed upper limit with respect to the interior pressure of the balloon. This degassing may result in inadequate inflation of the balloon.
If the balloon is left inside the patient's body in these undesirable situations, a blood clot is formed on the balloon surface, which may cause a serious side effect.
Japanese Patent Application Laid-open No. 5-192396 (hereinafter "JP '396") discloses a technique for gradually decreasing the opening time of the positive-pressure timing valve provided in the primary tube line in order to gradually reduce the medical aid given by the balloon catheter to the patient's heart beat as the patient's heart function recovers.
However, this technique cannot precisely control the contribution ratio of the balloon catheter to the patient's heart function because the patient's blood pressure changes every moment, and because the opening time of the valve varies due to the mechanical structure of the valve.
Generally, in the conventional driving apparatuses described above, the inflation/deflation rate of the balloon increases as the patient's heart beat rate increases and, consequently, the apparent pressure (i.e., the detected pressure) becomes lower than the actual pressure. In other words, when the inflation/deflation rate is high, the pressure of the secondary tube line starts decreasing before the actual pressure reaches the originally determined maximum inflation state. If the gas is supplied based on this information, an excessive amount of gas is introduced into the secondary tube line, which is not desirable for the patient or for the durability of the balloon.
To overcome this problem, the gas supply may be suspended for a while when the patient's heart beat rate is high. However, if such a situation continues for very long, the helium gas escapes from the secondary tube line through penetration and diffusion and the function of the balloon, to aid the heart beat rate of the patient, is lessened.
In some other conventional driving apparatuses, the entire gas in the secondary tube line is regularly replaced with new gas. In this case, the gas consumption increases, and if a relatively expensive helium gas is used as the driving gas, the apparatus becomes uneconomical. In addition, when substituting the gas, the balloon is not driven for several tens of seconds, during which the medical aid for the heart function of the patient is suspended.
Another problem of the conventional driving apparatuses using a pressure-transfer isolator is that a small peak appears in each pulse of the detected pressure of the secondary tube line. This peak prevents accurate detection of the interior pressure of the secondary tube line.